Pope Benedict XVI has suggested that the need to save mankind from a destructive blurring of gender roles is as important as saving the rainforests.
He explained that defending God's creation was not limited to saving the environment, but also about protecting man from self-destruction.
The Pope was delivering his end-of-year address to senior Vatican staff.
His words, later released to the media, emphasised his rejection of gender theory.
Speaking on Monday, Pope Benedict XVI warned that gender theory blurred the distinction between male and female and thus could lead to the "self-destruction" of the human race.
The speech has provoked anger from campaigners, who have interpreted the remarks as a call to save mankind from homosexuals and transsexuals.
Gender theory explores sexual orientation, the roles assigned by society to individuals according to their gender, and how people perceive their biological identity.
Gay and transsexual groups, particularly in the United States, promote it as a key to understanding and tolerance, but the Pope disagreed.
When the Roman Catholic Church defends God's Creation, "it does not only defend the earth, water and the air... but (it) also protects man from his own destruction," he said.
"Rainforests deserve, yes, our protection, but the human being ... does not deserve it less," the pontiff said.
It is not "out-of-date metaphysics" to "speak of human nature as 'man' or woman'", he told scores of prelates gathered in the Vatican's sumptuous Clementine Hall.
"We need something like human ecology, meant in the right way."
The Catholic Church opposes gay marriage. It teaches that while homosexuality is not sinful, homosexual acts are.
Rev Sharon Ferguson, chief executive of Britain's Lesbian and Gay Christian Movement, described the Pope's remarks as "totally irresponsible and unacceptable".
"When you have religious leaders like that making that sort of statement then followers feel they are justified in behaving in an aggressive and violent way," she said.
The pope uses his traditional end-of-year speech to offer his Christmas greetings and say a few words about what he considers the important issues of the day.
This year, Pope Benedict also deplored the tendency to depict the Catholic church's World Youth Day, which he attended in Sydney earlier this year, as mere spectacle.
He stressed that the event should not be considered a "variant of modern youth culture, as a kind of ecclesiastical rock festival with the Pope as the star," but as the fruition of a "long exterior and interior path".
This article has been amended to make it clear the Pope made no direct reference to homosexuals or transsexuals.